Kinds of Cell Division
Two types; mitosis and meiosis, Mitosis: cell division for growth and replacement wherein the two daughter cells are identical and similar to mother cell in all respects. Meiosis: the process of cell division in sexually reproducing organisms that reduces the number of chromosomes from diploid to haploid as in the production of gametes.
Mitosis divided into four phases or stages; Prophase, Metaphase, Anaphase and Telophase.
The nucleus divides first and then the whole cell divides. Division of one nucleus to give two daughter nuclei is termed as karyokinesis. Division of cytoplasm to give two daughter cells is termed as cytokinesis.
Prophase: three sub phases:
Early prophase: In early prophase, the chromosomes will appear as thin strands dispersed about the entire nucleus. The spindle apparatus will be forming but the thinness of the individual microtubule will probably make the spindle impossible to see at this early stage.
Middle prophase: the chromatin threads are now condensed enough to be distinguished as individual chromosomes. Each chromosome consists of two identical halves called chromatids that are connected and held together by a constriction called a centromere.
Late prophase: the nuclear membrane and the nucleolus finally disappear completely. The chromosomes are very distinct, easy to recognize and have clear "arms" composed of the two parts of the sister chromatids.
Chromosomes are lined up in one plate to form the equatorial plate. Each chromosome becomes attached to the spindle fibre by centromere. The sister chromatids are not yet separated. Smaller chromosomes are central in position whereas the larger ones are peripheral.
The centromeres of the chromosomes divide, two chromatids of each pair separate called as daughter chromosomes. The two sets of chromosomes migrate towards the poles. Chromosome movement is brought about by the shortening of spindle fibres attached to the centromeres.
During this phase, the sister chromatids reach opposite poles. The small nuclear vesicles in the cell begin to re-form around the group of chromosomes at each end. As the nuclear envelope re-forms by associating with the chromosomes, two nuclei are created in the one cell. Nucleolus becomes visible again.